refactor(index): single seam for resolving project root and opening the index

[ajianaz]

What

Evolves IndexBridge into the single seam for "resolve project root + open the index + ensure project id", and migrates all call sites to it.

Why

Closes #566. Callers answered "which project am I in?" differently: review-time scanners used raw current_dir() while cora index used resolve_project_root, so a review from a subdirectory or workspace member could resolve a different/empty project_id than the one indexed. PRAGMAs also differed per open path (6 vs 2).

How

• IndexBridge always normalises its start path with resolve_project_root. Modes: tolerant open (review; never creates the DB, never fails), strict open_strict (CLI/MCP; NoIndexError when absent), open_or_create (index/serve/watch/dead-code/routes/query), from_connection (tests).
• index::open_index_at + apply_pragmas are the only place PRAGMAs/migrations are applied; open_global_index and db_writer::open_db delegate to it. resolve_project_id removed.
• scan_unused_imports, scan_dead_code_in_review, scan_breaking_changes, scan_project_index, build_brain_context, build_scan_brain_context take &IndexBridge (the old scan_breaking_changes_with pattern, generalised) instead of opening their own connection.
• main.rs arms, mcp open_index_db, serve/watch/routes/query, review.rs and scan.rs go through the bridge.
• Behavior notes: the review path no longer creates an empty cora.db when none exists; the MCP/CLI "no index" message is unchanged for users.

Not included: index-session/config (#567), affected-tests (#568). Read-only db_writer::open_db_for_read (no PRAGMAs/migrations by design) and the context resolver's own IndexBridge::open(project_root) are left as is.

Testing

• cargo test --features tree-sitter passes
• cargo fmt --all -- --check passes
• cargo clippy --all-targets --features tree-sitter -- -D warnings passes
• cargo build --release --features tree-sitter not run locally (disk)
• New tests: tolerant open with absent index (no DB created), strict open errors with NoIndexError, subdirectory/workspace-member resolves same project_id as root, scanner run from a subdirectory finds callers indexed from root, PRAGMAs applied by the shared opener.

Closes #566

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